1. Field of Invention
This invention relates to electronic filters and in particular the tuning of gm-C filters.
2. Description of Related Art
Filters using transconductors and capacitors are often called "gm-C" filters. The transconductor is a circuit that has a voltage as an input and a current as an output. Most of the integrated gm-C filters use transconductance tuning or transconductance fixing with an external precision resistor. This method of tuning does not provide any compensation for process variations of the on chip capacitors; and therefore, the filter time constant is not process independent. There exists other methods of time constant tuning using phase locked loops that can keep the time constant tuning process independent, but these methods use analog circuits which require large silicon surfaces to implement and dissipate large amounts of power.
In U.S. Pat. No. 5,325,070 (McGinn) a transconductance gain amplifier is described that reduces the effects of temperature changes of on-chip resistors on the amplifier cut off frequency. In U.S. Pat. No. 5,384,501 (Koyama et al.) a differential amplifier is described in which a variable resistance in the form of a field effect transistor is connected between the differential input pair of the amplifier to provide a means to change the transconductance and the time constant of the differential amplifier. In U.S. Pat. No. 5,559,470 (Laber et al.) a high frequency continuous time filter using an active integrator and a transconductor stage is described which is insensitive to process variations and parasitic capacitance. The filter uses an external precision resistor to set the transconductance and a poly-fuse wafer-sort trim technique to remove capacitor process tolerances.
A simple method to achieve time constant tuning that is independent of process variations is needed that uses a variable mechanism that is independent of the circuitry producing the tuned time constant. The before mentioned prior art uses external resistance to adjust for process variations and to set the transconductance of the circuitry. An entirely integrated solution in which the transconductance and circuit time constant can be tuned by varying an external signal provides a means to achieve the tuning independent of process and process variations. To this end, the technique described in this invention provides a means to accomplish time constant tuning using simple circuitry without the use of phase locked loops, or time consuming polysilicon fuse trimming and subsequent testing.